As the header files state 1988 as last year, this would be around Microsoft C 5.0 or C 6.0 probably.

It's MSC 5.10, MASM 5.10, LINK 3.65, NMAKE 1.00.05, and some other stuff.

Comes down to around 2 MB after cleaning up a bit. 1.4 MB if only keeping support for one memory model. Runs fine in DOSBox, so this is an easy way to make a project self-contained, building from within the target platform with no external dependencies.